Optical discs are widely used as a medium for storing therein pieces of data of music, video, documents, or the like, and various devices for reading or writing information from/to an optical disc are developed. Such devices, in many cases, adopts an optical pickup device (See for example Japanese Unexamined Patent Publication No. 2003-187484 (Tokukai 2003-187484; Published on Jul. 4, 2003)) which device is a main element for inputting/outputting signals, by using its edge portion, to/from an optical disc.
FIG. 8 is a block diagram illustrating a main configuration of a conventional optical pickup device 801. The optical pickup device 801 includes: a semiconductor laser 85 serving as a light source; and a lens optical system 82 arranged in a light path between the semiconductor laser 85 and the optical disc 81, which system includes a prism 84 and a condensing lens 83. This optical pickup device 801 is configured so that laser light having reflected off the optical disc 81 reaches the prism 84 through the lens optical system 82. Then, the light is reflected off the prism 84, and enters a light-receiving amplifier circuit 91 having a light-receiving element (photodiode).
The semiconductor laser 85 outputs laser light, so as to irradiate the optical disc 81 with the laser light via the condensing lens 83. The laser light is then reflected off the optical disc 81, and reaches the prism 84 via the lens optical system 82. The laser light is reflected off the prism 84, and is input to the light-receiving amplifier circuit 91. In the light-receiving amplifier circuit 91, the laser light is subjected to photoelectric conversion, and is output, as electric signals, from the light-receiving amplifier circuit 91.
FIG. 9 is a circuit diagram illustrating an exemplary configuration of an equivalent circuit block of a conventional light-receiving amplifier circuit 91. The light-receiving amplifier circuit 91 includes a preamplifier circuit 92. The preamplifier circuit 92 includes an amplifier A91. To the input terminal of the amplifier A91 connected is one end of the light-receiving element PD whose another end is grounded. Furthermore, feedback resistors RfH91 and RfL91 each for current-voltage conversion are provided so that feedback resistors RfH91 and RfL91 and the amplifier A91 are connected in parallel to one another. One end of each of the feedback resistors RfH91 and RfL91 is commonly connected to the input terminal of the amplifier A91.
The light-receiving amplifier circuit 91 is provided with NPN transistors Q91 and Q92. The respective gates of the NPN transistors Q91 and Q92 are commonly connected to the output terminal of the amplifier A91. The respective collectors of the NPN transistors Q91 and Q92 are connected to a line via which a power source voltage Vcc is supplied. The emitter of the NPN transistor Q91 is connected to one end of a constant current source IcH91 whose another end is grounded. The emitter of the NPN transistor Q92 is connected to one end of a constant current source IcL91 whose another end is grounded.
Another end of the feedback resistor RfH91 is connected to the collector of the NPN transistor Q91, and another end of the feedback resistor RfL91 is connected to the collector of the NPN transistor Q92.
The light-receiving amplifier circuit 91 is further provided with a preamplifier circuit 93 whose configuration is the same as that of the preamplifier circuit 92. This preamplifier circuit 93 includes an amplifier A92 which is apart from the light-receiving element PD.
For the amplifier A92, feedback resistors RfH92 and RfL92 are provided so that feedback resistors RfH92 and RfL92 and the amplifier A91 are connected in parallel to one another. One end of each of the feedback resistors RfH92 and RfL92 is commonly connected to an input terminal of the amplifier A92.
The preamplifier circuit 93 is provided with NPN transistors Q93 and Q94. The respective gates of the NPN transistors Q93 and Q94 are commonly connected to the output terminal of the amplifier A92. The respective collectors of the NPN transistors Q93 and Q94 are connected to a line via which a power source voltage Vcc is supplied. The emitter of the NPN transistor Q93 is connected to one end of a constant current source IcH92 whose another is grounded. The emitter of the NPN transistor Q94 is connected to one end of a constant current source IcL92 whose another end is grounded.
Another end of the feedback resistor RfH92 is connected to the collector of the NPN transistor Q93, and another end of the feedback resistor RfL92 is connected to the collector of the NPN transistor Q94.
The light-receiving amplifier circuit 91 is provided with a differential amplifier circuit 98. The differential amplifier circuit 98 has a differential amplifier A93. One input terminal (Hereinafter, first input terminal) of the differential amplifier A93 is connected to a switch SW 91 via a resistor Rs91. To a connection point between the first input terminal of the differential amplifier A93 and the resistor Rs91 connected via a resistor Rf93 is a terminal through which an external reference voltage is supplied. Another input terminal (hereinafter, second input terminal) of the differential amplifier A93 is connected to a switch 92 via a resistor Rs92. Between the other input terminal of the differential amplifier A93 and the output terminal provided is a feedback resistor 94.
The switch SW91 is for switching the connection of the first input terminal of the differential amplifier A93 to one of (i) a terminal between the NPN transistor Q91 and a feedback resistor RfH91 and (ii) a terminal between the NPN transistor Q92 and a feedback terminal RfL91. A switch SW 92 switches the connection of the second input terminal of the differential amplifier A93 to one of (i) a terminal between the NPN transistor Q93 and the feedback resistor RfH92 and (ii) a terminal between the NPN transistor Q94 and the feedback resistor RfL92.
A laser light signal is converted into an electric-current signal Isc by the light-receiving element PD, and the electric-current signal Isc is subjected to current-voltage conversion and amplification by using the feedback resistor RfH91. Here, an output circuit of the preamplifier 92 illustrated in FIG. 9 is an emitter follower circuit which is configured by the NPN transistor Q91 and a constant current source IcH91.
The signal having been subjected to the current-voltage conversion by the preamplifier 92 is then multiplied by (Rf94/Rs92) by the differential amplifier circuit 98 in the later stage, and the resulting signal is output as an electric signal Vo. At this point, the constant current source IcL91 of the preamplifier circuit 92 is not operating. Only a loop which is connected to the constant current source IcH91 and which includes the feedback resistor RfH91, NPN transistor Q91, and amplifier A91 is operated. A loop which is connected to the constant current source IcL91, and which includes the feedback resistor RfL91, NPN transistor Q92, and the amplifier A91 is not operated.
This example deals with a case where two sensitivities (gains) are provided. However, it is possible to provide three or more sensitivities (gains) by increasing the number of feedback resistors, transistors, and amplifiers A91 each connected to a constant current source.
When a recording/reproducing-use optical pickup device writes in data, it forms pits on an optical disc by irradiating the optical disc with laser light of 200 mW or more. On the other hand, when the optical pickup device reads out data on an optical disc, the device irradiates an optical disc with less intensive laser light of approximately 20 mW, and reads out data recorded on the optical disc by reading variation in reflection of the laser light off the optical disc. Therefore, the light power for writing in data and the light power for reproducing data differ from each other by approximately a factor of 10. Further, the reflectance of optical discs differs amongst ROM disc, RAM disc, ±R disc, and ±RW disc by approximately a factor of 1 to 8. Accordingly, an amount of laser light incident on the light-receiving amplifier circuit of the optical pickup device largely varies depending on whether data is being written in or being read out, and depending on the type of optical disc being handled.
On this account, in the light-receiving amplifier circuit, plural sensitivities (gains) are switched thereamong in accordance with a variation range of incident light amount, so as to correspond to variation in the amount of laser light being incident. For example, when writing in data, the amount of laser light incident is large, and as such, the sensitivity (gain) of the light-receiving amplifier circuit is switched to a small sensitivity (gain). Meanwhile, when reading out data, the amount of laser light incident is small, and as such, the sensitivity (gain) of the light-receiving amplifier circuit is switched to a large sensitivity (gain).
The sensitivity (gain) of the light-receiving amplifier circuit 91 illustrated in FIG. 9 is determined by multiplying the feedback resistor RfH91 of the preamplifier circuit 92 by (Rf94/Rs92) by the differential amplifier circuit 98 in the later stage. In a case of reading out data from a less reflective optical disc such as RAM disc, the light-receiving amplifier circuit needs to be extremely sensitive. Therefore, the gain resistance (RfH91×Rf94/Rs92) of the light-receiving amplifier circuit 91 is large.
When the resistance values of the resistors RfH91 and Rf94 are large, noise in the light-receiving amplifier circuit 91 itself is also large. This causes an extremely poor S/N ratio of output signals Vo of the light-receiving amplifier circuit 91. The noise in the light-receiving amplifier circuit 91 itself is mostly heat noise which is expressed by the following formula:{4k(RfH91)T(Δf)}1/2×Rf94/Rs92.
In order to reduce such heat noise of the light-receiving amplifier circuit 91, it is necessary to reduce the resistance of the feedback resistor RfH91 in the preamplifier circuit 92, or to reduce the gain (Rf94/Rs92) of the differential amplifier circuit 98 of the subsequent stage. As is indicated by the above formula, the heat noise is proportional to the square root of the feedback resistor RfH91 of the preamplifier circuit 92. However, the gain (Rf94/Rs92) of the differential amplifier circuit 98 in the subsequent stage multiplies the heat noise by (Rf94/Rs92). Accordingly, it is more effective to reduce the heat noise by reducing the gain (Rf94/Rs92) of the differential amplifier circuit 98.
In the conventional light-receiving amplifier circuit 91, the gain (Rf94/Rs92) of the differential amplifier circuit 98 is set at 1 or more. In such a conventional circuit, however, the reduction of the gain (Rf94/Rs92) of the differential amplifier circuit 98 to less than 1 causes a problem of smaller output voltage range (dynamic range) of the light-receiving amplifier circuit, than a case where the gain is 1 or more.